I'm not sure if this is a proper use of this board, but I just spent a considerable amount of time trying to solve a problem that the Internet was no help for, and I figured that if anyone else ever has this problem, this post might save them some time. (The problem was in align, and the Internet's only suggestion was "use eqnarray instead".) I'm still just a n00b, though, so if this should go elsewhere or nowhere, or if the solution is obvious and I just need more experience, let me know.

If the command is defined, you will get an error. That's the usual way to define commands and to make sure not to overwrite something. There is another method to make sure that existing commands are not simply overwritten.

This will cause no error but only define the command if it doesn't already exist. Otherwise the definition is ignored.

Since you consider yourself a noob, you should not tinker around with stuff from which you don't know for sure what it really does. And TeX's \def command is certainly nothing for beginners. It overwrites commands without checking if the defined command already exists. So, hands off!